Recommended VMFS size for boot OS

[gselby]

Hi there,

I am creating individual VMFS LUNS for every VM OS. The OS has a boot partition of 10GB. All other drives are RDM mounts so are not relevant. I don't want to create one large VMFS volume for all boot OS's

I found that creating a 15GB VMFS volume leaves very little space once the VM boots up. I'm using 20GB volumes at the moment that seem to be fine for space (about 8-9GB free) but this space drops considerably when I start a snapshot. I intend using Consolidated Backups which use snapshots but the backup software consolidates the delta disk after the backup completes and then deletes the snapshot. I don't intend using snapshots for any other reason. Is there anything else I need to be aware of that can eat up space on the VMFS? My concern is that 20GB may not be large enough. On the other hand I don't want to waste space on the SAN by having (potentially) 100s of oversized VMFS volumes.

Any advice would be very appreciated!

Thanks in advance

Glenn

[mcowger]

And you've discovered the reason NOT to create a sep. VMFS for every VM. Eventually you are also going to run into the max number of LUNs/Paths.

--Matt

[bretti]

VM Swap file will also take up disk space.

[gselby]

To be honest I originally had one large VMFS but the boss wasn't happy with that and wanted seperates ones for every OS. I'm just following orders I'm afraid

As for max LUNs, we can get round that with multiple clusters

[gselby]

Does anyone kow what VMware official recommendation is for this? One big VMFS or individual ones?

[RParker]

Methinks the boss needs to let you do your job and quit micromanaging.... VMFS as a LARGE LUN makes better sense, especially since there is this tool... umm.. Can't remember what it's called.. oh yeah BACKUP! That's what its for, to restore VM's... It makes things eaiser as 1 LARGE VMFS, maybe he is old school in ASSUMING VMFS works the same way as NTFS which it doesn't....

Anyway.. using 1 LARGE VMFS/LUN isn't a problem.

Did you ask him specifically WHY he wants it that way? And "because I said so" isn't an answer, never was, and it certainly isn't now, it's a COP OUT. He doesn't know why or understand VMFS. I would like to know why a VMFS volume as 1 big LUN won't work, especially since your "boot" drives are more like application/boot/install/swap drives. A boot drive would be more like 2 or 3 gig, not 20.

[bretti]

Not sure what VMware's official recommendation is. Best practices show that there is a good middle ground. Definately do not create one LUN for each OS VMFS. On the other hand depending on the amount of storage you have, you probably don't want to create one BIG VMFS LUN.

There is usually a middle ground. Our middle ground ended up being multiple 300GB LUNS. This has been a good number for us.

[mcowger]

I dunno about official recommendation, but every implementation i've seen (and, in fact, the class) say 300-700GB LUNs. We use 534, and have excellent results.

--Matt

[gselby]

I originally had a 350GB LUN. The boss's concern is should someone accidentally remove the vlun on the SAN then we could potentially lose connectivity to multiple servers. He will always err on the side of caution where ever possible.

[mcowger]

Yeah, thats true, but your proper process and careful work will take care of that. If he is that scared, split it into 3 LUNs, but its hinestly silly.

--Matt

[gselby]

Hmm, not sure about splitting.

So going back to the original question. Do you reckon 20GB is big enough?

Cheers

[bretti]

20 GB will be big enough to do what you want to do.

With a 10 GB VMDK the biggest size a snapshot could be is 10GB which is not likely to happen. Plus however much you have for RAM that may go to swap files.

[gselby]

Cool.

Thanks

[Jae_Ellers]

First of all, you should know how big your OS footprint is. Then you need a space for swap. This is the sizeof the memory allocated unless you've set limits and reservations. If you have set these then the swap is the sizeof(limit - reservation). Then you need space for log files. These can be very large (for text files) and I've seen > 1GB logs when things are wrong such as deleted .iso files that are connected to cdroms.

So if you're comfortable with a 20 Gb boot disk and you have a 4 Gb RAM system I'd allocate something like 25-28 Gb.

I'd question the managment edict as unless you need a seperate lun for throughput or storage managment the typical lun is created for 10-25 vms, and the number thrown out before of 300-700 GB is a good guideline.

There are reasons for a lun per disk or vm, but unless you know why you need it, you don't need it and are creating a huge unnecessary managment overhead that would make it more likely that something important is lost in the shuffle. Which is easier? Keeping track of 15x 500 Gb luns or 300x 25 Gb luns? With the 15 luns you can easily present all luns to all servers and not have to worry about the cluster zoning.

Currently the accepted best practice is just that, creating multiple VMs per lun. Also consider the hassle of resizing virtual disks, should the need arise.

Time to push back.

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[gselby]

Thanks Jae. I shall put these recommendations to the boss and see what he says.

[wondab]

I think it's a bad idea to create 20 GB LUNs, with lots of little LUNs it will eventually become an adminsitrative nightmare from the SAN side.

And as mentioned before, swap needs space and what if you want to grow out a disk? If someone would delete a LUN which is very unlikely, you surely have backups. Depending on how big your environment is I would go with at least 300 GB per LUN. Just my opinion cause in the event of having to present and unpresent your LUNs it can be a real pain. I use 1 and 1.5 TB LUNs.